In semiconductor fabrication, a substrate or wafer is subjected to a number of processes in order to deposit or remove a layer from the wafer. Many of these processes involve the introduction of gases into a reaction chamber containing the wafer. These gases can include, for example, reactive gases introduced to deposit a layer on the substrate. In addition, inert gases can also be introduced to purge reactive gases from the chamber between reactive steps. Systems for introducing gases generally include gas sources (such as gas tanks, bubblers, other liquid and/or solid vaporization devices) connected via piping ultimately to the reaction chamber. Mass flow controllers and valves are adjusted in order to select the type and amount of the desired source gas(es) to introduce into the reaction chamber. Each mass flow controller (“MFC”) is normally calibrated to measure the flow rate of a particular type of process gas at a particular, flow rate or range of flow rates.
Semiconductor manufacturers have a continuing need to improve process accuracy and repeatability, both on a single tool, and across all tools in a fabrication operation running the same process. Deviations are caused by many factors, but a prime cause of deviation is the accuracy and repeatability of process gas flows controlled by MFCs. Any deviation from the idealized perfect flow for a particular process (caused by an inaccurate MFC, for example) can cause loss of yield and increased costs, and can affect the quality of the finished semiconductor. An MFC whose actual flow rate deviates from its intended flow rate (this deviation is sometimes referred to as “drift”) must be recalibrated or replaced, which leads to tool down time, decreased output and increased costs.
Some digitally controlled MFCs have accuracy levels reportedly in the range of +/−1% of the desired flow rate when the MFC is new. Devices such as MFVs (mass flow verifiers) or MFMs (mass flow meters) can be used to monitor and verify that the amount of gas entering a tool as measured by an MFC is the actual amount entering the tool, i.e., they verify the MFCs accuracy. MFVs generally have an accuracy of +/−1% and can be used to monitor and verify whether the amount the gas flow rate measured by an MFC has deviated from the actual gas flow through the MFC. However, MFV use is very time consuming, taking up to several days to perform a complete verification. Such a verification is typically performed quarterly, thus not always detecting MFC deviations early enough to avoid the afore-mentioned problems with drift.
MFMs can also be used to monitor MFC drift and can detect drift much faster. MFCs and MFMs, however, are configured for a particular gas type and a particular flow rate range. It is impractical to include a duplicately configured MFM for each gas type and flow rate range for which each MFC is configured.
There also has not been a way to improve the accuracy of many tools in a process fabrication using these validation tools, but only the tool within which the MFM or MFV resides.
A method to verify the actual gas flow through an MFC and reduce process deviation caused by MFC variability, whether for a single tool or from tool to tool within a semiconductor fabrication facility, is highly desired.